Covalent Bonding

Sharing electrons, Lewis dots, geometry, and polarity.

Dr. Melinda Oliver

CHM 101

Naming Binary Covalent Compounds

You can use the prefixes now! (Binary = two elements) We can call most covalently bonded substances Molecular! This is not true for ionic compounds (no true molecules)

  • Nonmetal to the left on PT comes first in formula and in name. Do not change name. You will see exceptions to this "rule" especially for hydrogen.
  • Use the subscripts in the formula to determine the prefixes. The figure to the right has the prefixes from 1-10. Most of these are familiar.
  • If the element to the left on the PT, and, hence, in the formula, has no subscript, which means there is only one of this element in the formula, the prefix mono- is not used.
  • The other nonmetal comes second in formula and in name. Drop the ending on this one and add the suffix -ide. So pretty much the same as ionic compounds.
  • Do not try to figure out charges as these elements are sharing.
  • Example: CO2 is carbon dioxide. Carbon is on the left and there is only one C in the formula (so no mono- prefix). N2O4 is dinotrogen tetroxide. We do not usually put two vowels next to ech other so that is why it is not tetraoxide.

Examples on naming are done along with Lewis dot structures, see below for examples.

Naming Acids

  • Most acids are compounds that have a hydrogen atom bonded to a highly electronegative atom so that the hydrogen can be "taken" by the action of hydrogen bonding with another molecule like water. When this happens, hydrogen has no electrons and we call it a "proton" because it is essentially just a proton (because the most common isotope of hydrogen has no neutrons).
  • Binary acids (H and another element) are named use the hydro- prefix and the -ic ending for the other element. For example, HBr is hydrobromic acid and H2S is hydrosulfuric acid.
  • Acids that have hydrogen and oxygen are the so-called oxyacids and their names come from the polyatomic ions that combine with hydrogen to make the acid. If the polyatomic ion ends with -ate like sulfate, nitrate, etc become -ic in the acid form.
  • Polyatomic ions ending in -ite like nitrite become -ous in the acid form. So nitrous acid has the formula, HNO2 and H2SO3 is named sulfurous acid.

Pg 13 in the notes is covered in Lecture 9 at 12:56 min into the lecture.

Bonding Models and Lewis Structures: Crash Course Chemistry #24

Lewis Dots

Follow these simple steps to draw Lewis dot structures: (do not micromanage electrons!)

  • Add up valence electrons for all atoms in the molecule. Add for negative/subtract for positive charges (polyatomic ions).
  • Connect atoms with bonding “dashes” (single bonds) indicating sharing of electrons.
  • Give each atom 8 electrons EXCEPT FOR B and H. (Li and Be are exceptions too but not seen often as they are METALS).
  • Count electrons.
  • If there are too many, look for C, N, O, P, and S atoms. If these atoms are present they can share more than 2 electrons in a multiple bond WITH EACH OTHER. Sharing 4 is a double bond. Sharing 6 is a triple bond.
  • If there are too few, the central atom can share more than 8 IF THAT ATOM IS 3RD ROW OR BELOW ON THE P.T.

The last part of lecture 8 and the first part of lecture 9 (first 13 min) goes over all of the examples in the notes.

Polar & Non-Polar Molecules: Crash Course Chemistry #23

Examples at the top on pg 28 in the notes begin at 32:28 in lecture 9.

Geometry Chart and Polarity

This chart will help you determine molecular polarity where I have written in "yes" and "no" on the sheet to indicate whether the geometry cancels out polar bonds. Follow the 3 steps above first as failure to do so can result in a wrong answer. What do the "yes" and "no" mean?

A “No” on the geometry sheet means that the geometry DOES cancel the polarity of the bonds and the molecule is, therefore, nonpolar. A “Yes” means that the polarity is NOT CANCELED and the molecule is, therefore, polar.

*IMPORTANT: The “Yes” and “No” on the geometry sheet are for molecules that have all bonds the same around the central atom like CF4 and CO2 but if the molecule is CH3F, all bonds are not the same and the polarities of the C-F and the C-H bonds are different so they do not cancel.

Resonance examples in Lecture 9

Examples in notes on pg 30 on resonance begin at 56:41 in lecture 9. Also, at the very beginning of lecture 10, another example is done.

What can you do to help your understanding?

Chemistry is a subject that must be practiced everyday if possible. Work through the lecture examples stopping the video clips and then restarting to check yourself. Take advantage of the online homework in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Sakai everyday and write down due dates from Mastering Chemistry. Keep a printed copy of the most recent course calendar (included in syllabus) next to your work area. Email me with questions!!